The manufacturers of semiconductor devices often demand phosphorus doped single crystal material having a very tight tolerance for resistivity with a distribution of the dopant as homogeneous as possible. The manufacturers of high-blocking thyristors are interested in such materials because it effects an optimization of the device properties. In contrast to conventional doping methods, it is possible to accomplish the desired doping conditions by means of neutron irradiation. The exact resistivity value can be obtained within an extremely narrow limit and the material is practically free from the macroscopic and microscopic variations of the electrical resistivity observed in conventionally doped semiconductor materials.
Neutron transmutation doping is the nuclear conversion of semiconductor materials atoms into dopant, i.e., silicon atoms into phosphorus dopant atoms by exposing undoped silicon crystals to a suitable flux of thermal neutrons in the core of a nuclear reactor. The nuclear reaction involved is: EQU .sup.30 Si+n.sub.th .fwdarw..sup.31 Si+.gamma..fwdarw..sup.31 P+.beta..sup.- (t.sub.1/2 =2.6 hrs)
The advantage of this technique is the chance to fabricate N-doped silicon of extreme homogeneity which is impossible to realize by conventional doping methods.
Most commercial uses of semiconductor materials require single crystal silicon of very accurately controlled resistivity. In certain instances, this requirement arises from the nature of the silicon devices, i.e., the avoidance of marked resistivity variation affecting electrical characteristics. In other words, when the device does not give rise to this requirement, characteristics of the starting material may determine the position and nature of junctions and gradients produced during manufacture, so that initially uniform properties in the starting material are necessary.
In most semiconductor materials constant resistivity is not easily obtained. The distribution coefficients for most significant impurities in, for example, silicon and the increased activity of silicon at its high melting point complicates the problem. Uniform P-type silicon has been produced by either of two methods. Using boron with a distribution coefficient of approximately 0.9, uniform crystals have been produced by crystal pulling. Using the float zone technique with aluminum, having a distribution coefficient of 0.004 P-type silicon of a high degree of uniformity has been produced by zone leveling.
The preparation of uniform resistivity N-type silicon has been a more difficult problem. The usual doping impurities of group V of the periodic chart do not lend themselves to use in conventional processing techniques. Certain of these impurities, such as antimony, are sufficiently volatile at the melting point of silicon that it is difficult to control their concentration during the usual crystal growing procedures. The less volatile group V impurities, phosphorus and arsenic, have distribution coefficients which are too small to permit uniform resistivity.
Phosphorus has been the group V element preferred for the doping of semiconductor grade silicon to give "N" type electron conductivity, especially for high resistivity materials for power devices. Since the segregation coefficient of phosphorus is relatively low, (0.35), a non-planar freezing interface during the growth of mono-crystals by the float zone process will cause a non-uniform radial distribution of phosphorus in the crystal. Since the center freezes last, it will contain more dopant than the edge. Minimizing this non-uniformity, the consequence of which is usually referred to as radial resistivity gradients, has been a goal of research in the zone-refining process. In the past few years, development of the spreading resistance probe method for measuring resistivity has revealed that numerous micro non-homogeneities exist in addition to the macro variations revealed by radial gradient measurements. It is generally agreed that the variations in dopant concentration in the crystal lattice play a major role in limiting the performance obtained in high power silicon devices and in reducing the yield in device manufacturing process.
From the view of large scale production, various aspects have to be considered like limitations due to residual radioactivity, reactor performances, irradiation capacity, requirements of the starting material, and annealing treatments. Since neutron transmutation doping can create only donors, and is hence used for applications in which the desired conductivity of the silicon is negative, most such applications specified not only the conductivity type, but also the number of donors that must be present in the silicon within narrow limits. The starting material for neutron transmutation doping, undoped single crystal silicon, will contain some donors and also some acceptors (dopant atoms that contribute an electrical hole to the conductivity, and result in positive conductivity) from the so-called base-level impurities in the polycrystalline silicon from which the single crystal silicon is produced.
One restriction in the selection of starting material for undoped single crystal silicon is that the number of donors in the material must be smaller than the number desired in the product, i.e. neutron doped silicon. Another requirement is that the number of acceptors in the undoped single crystal silicon not exceed a certain fraction of the number of donors. A third requirement is that the uniformity of the starting material be within certain limits, since any inhomogeneity in the starting material will show up as inhomogeneity in the neutron doped silicon.
Presently, the art taught methods of selecting "N" doped single crystal silicon for neutron transmutation doping specify that the starting material by ot one type only, and maximum donor or acceptor concentrations must be present at a level of only one-tenth of that desired in the neutron doped silicon product. While this rule ensures good uniformity in the product, it also results in the exclusion of up to 25% or more of all starting undoped single crystal silicon produced for neutron transmutation doping. Moreover, any material that misses the target cannot be recycled, either for the same specification or for a specification requiring additional donors; thus, the material is lost as scrap.